Microwave communication systems include one or more satellites receiving signals transmitted to it by an earth station. The satellites amplify and send this information to other earth stations on new carrier frequencies. A frequency difference of about 2 GHz prevents interference between the uplink and downlink transmissions. For example, all geostationary satellites operate in one of the following three bands:
______________________________________ Old Band Uplink Downlink Orbit Separation ______________________________________ C 6 GHz 4 GHz 4 degrees Ku 14 GHz 12 GHz 3 degrees K 17 GHz 12 GHz Not assigned ______________________________________
In certain earth locations such as the United States the communication systems operate at C band; while, in Europe the communication systems operate at Ku band. It is becoming increasingly desirable for earth stations to receive the programs of both the C band and Ku band.
Known earth stations include a parabolic (dish) reflector for collecting the microwave energy transmitted by the satellite. The dish focuses the reflected energy on a feedhorn assembly located at a focal point in front of the dish. An entire feedhorn assembly typically includes a feedhorn, a section of waveguide, a polarizer, and a low noise amplifier (LNA) plus associated cable. The LNA circuitry includes a power module for protecting the circuit against power surges or spikes. The power module is typically included in the LNA package which adds to the bulk and weight of the feedhorn assembly as well as to the heat generated in the LNA package. The heat dissipated during a power surge can destroy the LNA which it was designed to protect.
The microwave energy transmitted by satellites typically is polarized vertically and horizontally to double the number of transponders available. A good example of the use of dual polarization on a satellite is the RCA Statcom IIIR which operates at C band (4 GHz) with 24 transponders. The twelve odd-numbered transponders utilize the vertically polarized electric field, and the twelve even-numbered transponders utilize the horizontally polarized electric field. Polarizers increase substantially power insertion losses.
At an earth station receiving site it is necessary to adjust the polarization of the receiving antenna to correspond to the polarization of the set of transponders generating the desired signals in order to receive those signals. Some earth station antennae have dual polarized feeds which are capable of receiving both polarizations simultaneously and thus can receive any or all of the 24 transponders with no further adjustment of the antenna feed. Such dual systems, however, are very expensive which prohibits their use in the private segment of the commercial market. Nevertheless, even for this application, the antennae should be capable of receiving television programs from all of the satellites and from all of the transponders on each of the satellites. Thus, for best results (pictures) the antenna must be capable of responding to either horizontal polarization or vertical polarization of the frequency bands being used, namely, the C and Ku bands. Also, some satellites may have their polarizations skewed from either the vertical or horizontal positions. In this case the antenna must be positioned to respond to the signals having skewed polarizations.
Early earth station designs utilized a motor to rotate the entire feed assembly. The motor is controlled by the operator to position the feed assembly such that its polarization coincides to that of the transmitting satellite. However, the feed assembly was bulky and heavy; thus, rotation of the feed assembly without wobble by the motor drive was difficult. Any wobble of the feedhorn during rotation caused the antenna beam to depart from true boresight along the focal axis, and the signal from the satellite was not in the maximum of the receiving antenna pattern. To alleviate the wobble problem, efforts were directed toward obtaining the desired polarization using a stationary feed assembly. In addition, wind forces result in decreased aiming accuracy and a loss of the incoming signals.
These efforts included the use of a septum in the rotating waveguide. A septum is a metal plate positioned across the waveguide. The lines of an electric field are all normal to a plane which passes horizontally through the center of the waveguide. In a circular waveguide the plane is the horizontal diameter. When properly aligned, the septum will not block or attenuate the wave nor will it cause reflections to occur so long as it is a relatively thin conducting sheet. The septum can be of any length, and the wave as it travels through the guide will reform after it has passed by the septum into a wave identical to the original wave. In effect the electric field lines being normal to the septum do not see the septum, and the wave is said to be cross polarized with respect to the septum.
Another form of the septum included spaced diametric conducting pins mounted across the diameter of the circular waveguide in the same plane as the previously described septum, and spaced along the longitudinal axis of the guide in relatively close proximity (small fractions of a wavelength) one to another. Each pin was slightly rotated a few degrees (only enough to prevent discontinuities) and a gradual rotation of the polarization began without upsetting the wave propagation in the waveguide. If the pins themselves are rotated as described in U.S. Pat. Nos. 3,287,729 and 3,296,558, the entire feed assembly need not be rotated.
To avoid the need for a complex pin rotational mechanism, a twistable serpentine-shaped filament was developed. The filament comprises a series of interconnected legs for transverse orientation to wave propagation at the diameter of a circular waveguide. Each leg is approximately equal in length but slightly less than the diameter of the waveguide. The filament terminates in a leg at each end. One end leg is rigidly mounted to the wall of the desired waveguide input to the LNA, and the other end is securely fastened to a rotatable sleeve for rotation around the longitudinal axis of the waveguide. Thus, the only driven element is the leg nearest the aperture of the feed. The serpentine shape of the filament assures accurate leg-to-leg spacing and successively small progression of leg-to-leg rotation. By appropriate selection of a resilient material, rotation of the legs of the filament is repeatable. More information about the serpentine filament is given in U.S. Pat. No. 4,503,379.
The disadvantage of the above-described feed assembly structures include their rotational-prohibitive size and weight, the substantial power insertion loss attending the use of septums as polarizering elements, heat destruction of the low noise amplifier (LNA) or low noise "block" (LNB) or module resulting from including the power regulator within the LNA or LNB where heat generated by regulating high voltages or transients destroys not only the power regulator but also the LNA or LNB; and decreased aiming accuracy attending the narrow half power beamwidth produced by these systems. A LNB is a LNA combined with a frequency downconverter and IF amplifier for producing modulated IF signals.